1. Field of the Invention
The present invention relates to a semiconductor laser device and a method of driving the same. More particularly, the present invention relates to a semiconductor laser device capable of emitting laser lights of different wavelengths in response to a change in the level of a driving electrical current supplied to the device, and also to a method of driving such a semiconductor laser device.
2. Description of the Related Art
In recent years, there is a rapidly increasing demand for semiconductor laser devices in the fields of optical communication and optical information processing, requiring a variety of functions to be performed by such devices. One of such needs is to provide a semiconductor laser device capable of varying emission wavelength. In a system for recording and reproducing signals to and from a medium such as an optical card or disk by means of a laser beam, it is a common measure to set the power of the reproducing beam at a level lower than that for recording, in order to prevent undesirable destruction of the recorded information by the reproducing beam. If the recording and reproduction are conducted by a single semiconductor laser device such that the wavelength of the reproducing light is set in a region where the medium is comparatively insensitive to the medium, it is possible to conduct reproduction without restricting the recorded information and without substantially reducing the reproduction output level, thus enabling a reproduction of information at a high S/N ratio.
Many proposals have been made to comply with such a requirement. For instance, Appl. Phys. Lett. Vol. 36, p. 442 (1980) proposes a semiconductor laser device of a first type in which a plurality of light-emitting layers capable of emitting laser beams of different wavelengths are formed on a single substrate but in different light waveguides, the laser being independently and selectively supplied with different levels of driving currents so that the single semiconductor laser device can emit laser beams of different wavelengths. This semiconductor laser device is therefore a combination of a plurality of independent laser devices formed on a single substrate.
A semiconductor laser device of a second type called "distributed Bragg reflection type (DBR) semiconductor laser" is also known in which a grating is used as the reflector which constitutes a resonator. This semiconductor laser device has an additional electrode provided on the grating to enable injection of carriers, i.e., supply of an electrical current. By controlling the rate of supply of the electrical current, the refraction index of the grating is changed so as to vary the emission wavelength. The construction of the light emitting layer or the like of this semiconductor laser is substantially the same as that of ordinary semiconductor lasers.
J. APPl. Phys. vol. 64, p. 1022 (1988) proposes a third type semiconductor laser device in which a single quantum well is used as an emission layer, wherein emission is made possible also from a quantum level of a higher order by increasing resonator loss, whereby laser beams of different wavelengths are obtainable at a first quantum level and a second quantum level.
The specification of U.S. Pat. No. 4,817,110 discloses a semiconductor laser device of a fourth type in which a pair of quantum wells having emission wavelengths which are not so much different from each other are used as light-emitting layers to emit laser beams of different wavelengths.
These known semiconductor laser devices have the following disadvantages.
In the semiconductor laser device of the first known type, laser beams of different wavelengths are emitted from different positions on the semiconductor laser. Therefore, when an external optical system to be combined with this semiconductor laser device is designed to focus a beam of a specific wavelength at a single fixed focal point, the laser beams of other wavelengths are fixed at points which are deviated from the above-mentioned focal point in amounts which are much greater than the slight deviation caused by wavelength dispersion. In addition, since a plurality of independent lasers are formed on the same substrate and driven independently, the production process is complicated and the size of the device is increased undesirably.
In the second known type semiconductor laser device, the wavelength is variable only in a restricted wavelength region. For instance, a laser of Al.sub.x Ga.sub.1-x As type laser exhibits a very narrow wavelength region of several nm. This is because the photo-gain of the light-emitting layer can have a value large enough for emission only in such a restricted wavelength region.
The semiconductor laser device of the third known type cannot provide high laser efficiency because this type of device relies upon an increase in the loss in resonance. In consequence, emission threshold current is increased to make it impossible to obtain a large output power. Therefore, it has been impossible to conduct double-wavelength continuous emission at room temperature with this type of device.
In the semiconductor laser device of the fourth known type, the difference between two different wavelengths is very small: namely, the emission wavelength is variable only within a narrow region.